Most fuel injection systems, either single point or multipoint systems, use electromagnetic fuel injectors for controlling the flow of fuel into the engine. The amount of lift, the actual opening height of the valve, is directly proportional to the working air gap between the pole piece and the armature of the solenoid controlling the movement of the valve. The force of the solenoid is proportional to the square of the distance between the pole and the armature. The tolerance of the lift dimension of fuel injectors is plus or minus two ten thousandths of an inch (0.0002") (0.005 mm), therefore, very precise control of the working air gap of the solenoid is required.
One of the more common means of accurately setting the lift of an injector is the placement of a precision ground spacer between the injector housing assembly and the valve body assembly. The spacer thickness is determined by accurately measuring the armature and the pole piece relative to axially spaced and aligned surfaces. From a comparison of these two measurements and with the addition of the measurement representing the desired lift, a ground spacer is added at assembly.
This operation requires the stockpiling of several different sizes of pre-ground spacers to be available during the assembly of the injectors resulting in hand assembly of each of the injectors and the resultant highly labor intensive product.